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<body lang="en">
<a name="Using-Variables"></a>
<div class="header">
<p>
Next: <a href="Conditionals.html#Conditionals" accesskey="n" rel="next">Conditionals</a>, Previous: <a href="Recipes.html#Recipes" accesskey="p" rel="prev">Recipes</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="How-to-Use-Variables"></a>
<h2 class="chapter">6 How to Use Variables</h2>
<a name="index-variable"></a>
<a name="index-value"></a>
<a name="index-recursive-variable-expansion"></a>
<a name="index-simple-variable-expansion"></a>

<p>A <em>variable</em> is a name defined in a makefile to represent a string
of text, called the variable&rsquo;s <em>value</em>.  These values are
substituted by explicit request into targets, prerequisites, recipes,
and other parts of the makefile.  (In some other versions of <code>make</code>,
variables are called <em>macros</em>.)
<a name="index-macro"></a>
</p>
<p>Variables and functions in all parts of a makefile are expanded when
read, except for in recipes, the right-hand sides of variable
definitions using &lsquo;<samp>=</samp>&rsquo;, and the bodies of variable definitions
using the <code>define</code> directive.
</p>
<p>Variables can represent lists of file names, options to pass to compilers,
programs to run, directories to look in for source files, directories to
write output in, or anything else you can imagine.
</p>
<p>A variable name may be any sequence of characters not containing
&lsquo;<samp>:</samp>&rsquo;, &lsquo;<samp>#</samp>&rsquo;, &lsquo;<samp>=</samp>&rsquo;, or whitespace.  However, variable names
containing characters other than letters, numbers, and underscores
should be considered carefully, as in some shells they cannot be
passed through the environment to a sub-<code>make</code>
(see <a href="Recipes.html#Variables_002fRecursion">Communicating Variables to a
Sub-<code>make</code></a>).  Variable names beginning with &lsquo;<samp>.</samp>&rsquo; and an
uppercase letter may be given special meaning in future versions of
<code>make</code>.
</p>
<p>Variable names are case-sensitive.  The names &lsquo;<samp>foo</samp>&rsquo;, &lsquo;<samp>FOO</samp>&rsquo;,
and &lsquo;<samp>Foo</samp>&rsquo; all refer to different variables.
</p>
<p>It is traditional to use upper case letters in variable names, but we
recommend using lower case letters for variable names that serve internal
purposes in the makefile, and reserving upper case for parameters that
control implicit rules or for parameters that the user should override with
command options (see <a href="Running.html#Overriding">Overriding Variables</a>).
</p>
<p>A few variables have names that are a single punctuation character or
just a few characters.  These are the <em>automatic variables</em>, and
they have particular specialized uses.  See <a href="Implicit-Rules.html#Automatic-Variables">Automatic Variables</a>.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="#Reference" accesskey="1">Reference</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to use the value of a variable.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Flavors" accesskey="2">Flavors</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Variables come in two flavors.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Advanced" accesskey="3">Advanced</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Advanced features for referencing a variable.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Values" accesskey="4">Values</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">All the ways variables get their values.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Setting" accesskey="5">Setting</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to set a variable in the makefile.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Appending" accesskey="6">Appending</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to append more text to the old value
                                  of a variable.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Override-Directive" accesskey="7">Override Directive</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to set a variable in the makefile even if
                                  the user has set it with a command argument.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Multi_002dLine" accesskey="8">Multi-Line</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">An alternate way to set a variable
                                  to a multi-line string.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Undefine-Directive" accesskey="9">Undefine Directive</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">How to undefine a variable so that it appears
                                  as if it was never set.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Environment">Environment</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Variable values can come from the environment.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Target_002dspecific">Target-specific</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Variable values can be defined on a per-target
                                  basis.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Pattern_002dspecific">Pattern-specific</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Target-specific variable values can be applied
                                  to a group of targets that match a pattern.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Suppressing-Inheritance">Suppressing Inheritance</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Suppress inheritance of variables.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Special-Variables">Special Variables</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Variables with special meaning or behavior.
</td></tr>
</table>

<hr>
<a name="Reference"></a>
<div class="header">
<p>
Next: <a href="#Flavors" accesskey="n" rel="next">Flavors</a>, Previous: <a href="#Using-Variables" accesskey="p" rel="prev">Using Variables</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Basics-of-Variable-References"></a>
<h3 class="section">6.1 Basics of Variable References</h3>
<a name="index-variables_002c-how-to-reference"></a>
<a name="index-reference-to-variables"></a>
<a name="index-_0024_002c-in-variable-reference"></a>
<a name="index-dollar-sign-_0028_0024_0029_002c-in-variable-reference"></a>

<p>To substitute a variable&rsquo;s value, write a dollar sign followed by the name
of the variable in parentheses or braces: either &lsquo;<samp>$(foo)</samp>&rsquo; or
&lsquo;<samp>${foo}</samp>&rsquo; is a valid reference to the variable <code>foo</code>.  This
special significance of &lsquo;<samp>$</samp>&rsquo; is why you must write &lsquo;<samp>$$</samp>&rsquo; to have
the effect of a single dollar sign in a file name or recipe.
</p>
<p>Variable references can be used in any context: targets, prerequisites,
recipes, most directives, and new variable values.  Here is an
example of a common case, where a variable holds the names of all the
object files in a program:
</p>
<div class="example">
<pre class="example">objects = program.o foo.o utils.o
program : $(objects)
        cc -o program $(objects)

$(objects) : defs.h
</pre></div>

<p>Variable references work by strict textual substitution.  Thus, the rule
</p>
<div class="example">
<pre class="example">foo = c
prog.o : prog.$(foo)
        $(foo)$(foo) -$(foo) prog.$(foo)
</pre></div>

<p>could be used to compile a C program <samp>prog.c</samp>.  Since spaces before
the variable value are ignored in variable assignments, the value of
<code>foo</code> is precisely &lsquo;<samp>c</samp>&rsquo;.  (Don&rsquo;t actually write your makefiles
this way!)
</p>
<p>A dollar sign followed by a character other than a dollar sign,
open-parenthesis or open-brace treats that single character as the
variable name.  Thus, you could reference the variable <code>x</code> with
&lsquo;<samp>$x</samp>&rsquo;.  However, this practice is strongly discouraged, except in
the case of the automatic variables (see <a href="Implicit-Rules.html#Automatic-Variables">Automatic Variables</a>).
</p>
<hr>
<a name="Flavors"></a>
<div class="header">
<p>
Next: <a href="#Advanced" accesskey="n" rel="next">Advanced</a>, Previous: <a href="#Reference" accesskey="p" rel="prev">Reference</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="The-Two-Flavors-of-Variables"></a>
<h3 class="section">6.2 The Two Flavors of Variables</h3>
<a name="index-flavors-of-variables"></a>
<a name="index-recursive-variable-expansion-1"></a>
<a name="index-variables_002c-flavors"></a>
<a name="index-recursively-expanded-variables"></a>
<a name="index-variables_002c-recursively-expanded"></a>

<p>There are two ways that a variable in GNU <code>make</code> can have a value;
we call them the two <em>flavors</em> of variables.  The two flavors are
distinguished in how they are defined and in what they do when expanded.
</p>
<a name="index-_003d"></a>
<p>The first flavor of variable is a <em>recursively expanded</em> variable.
Variables of this sort are defined by lines using &lsquo;<samp>=</samp>&rsquo;
(see <a href="#Setting">Setting Variables</a>) or by the <code>define</code> directive
(see <a href="#Multi_002dLine">Defining Multi-Line Variables</a>).  The value you specify
is installed verbatim; if it contains references to other variables,
these references are expanded whenever this variable is substituted (in
the course of expanding some other string).  When this happens, it is
called <em>recursive expansion</em>.
</p>
<p>For example,
</p>
<div class="example">
<pre class="example">foo = $(bar)
bar = $(ugh)
ugh = Huh?

all:;echo $(foo)
</pre></div>

<p>will echo &lsquo;<samp>Huh?</samp>&rsquo;: &lsquo;<samp>$(foo)</samp>&rsquo; expands to &lsquo;<samp>$(bar)</samp>&rsquo; which
expands to &lsquo;<samp>$(ugh)</samp>&rsquo; which finally expands to &lsquo;<samp>Huh?</samp>&rsquo;.
</p>
<p>This flavor of variable is the only sort supported by most other
versions of <code>make</code>.  It has its advantages and its disadvantages.
An advantage (most would say) is that:
</p>
<div class="example">
<pre class="example">CFLAGS = $(include_dirs) -O
include_dirs = -Ifoo -Ibar
</pre></div>

<p>will do what was intended: when &lsquo;<samp>CFLAGS</samp>&rsquo; is expanded in a recipe,
it will expand to &lsquo;<samp>-Ifoo -Ibar -O</samp>&rsquo;.  A major disadvantage is that you
cannot append something on the end of a variable, as in
</p>
<div class="example">
<pre class="example">CFLAGS = $(CFLAGS) -O
</pre></div>

<p>because it will cause an infinite loop in the variable expansion.
(Actually <code>make</code> detects the infinite loop and reports an error.)
<a name="index-loops-in-variable-expansion"></a>
<a name="index-variables_002c-loops-in-expansion"></a>
</p>
<p>Another disadvantage is that any functions
(see <a href="Functions.html#Functions">Functions for Transforming Text</a>)
referenced in the definition will be executed every time the variable is
expanded.  This makes <code>make</code> run slower; worse, it causes the
<code>wildcard</code> and <code>shell</code> functions to give unpredictable results
because you cannot easily control when they are called, or even how many
times.
</p>
<p>To avoid all the problems and inconveniences of recursively expanded
variables, there is another flavor: simply expanded variables.
</p>
<a name="index-simply-expanded-variables"></a>
<a name="index-variables_002c-simply-expanded"></a>
<a name="index-_003a_003d"></a>
<a name="index-_003a_003a_003d"></a>
<p><em>Simply expanded variables</em> are defined by lines using &lsquo;<samp>:=</samp>&rsquo;
or &lsquo;<samp>::=</samp>&rsquo; (see <a href="#Setting">Setting Variables</a>).  Both forms are
equivalent in GNU <code>make</code>; however only the &lsquo;<samp>::=</samp>&rsquo; form is
described by the POSIX standard (support for &lsquo;<samp>::=</samp>&rsquo; was added to
the POSIX standard in 2012, so older versions of <code>make</code> won&rsquo;t
accept this form either).
</p>
<p>The value of a simply expanded variable is scanned
once and for all, expanding any references to other variables and
functions, when the variable is defined.  The actual value of the simply
expanded variable is the result of expanding the text that you write.
It does not contain any references to other variables; it contains their
values <em>as of the time this variable was defined</em>.  Therefore,
</p>
<div class="example">
<pre class="example">x := foo
y := $(x) bar
x := later
</pre></div>

<p>is equivalent to
</p>
<div class="example">
<pre class="example">y := foo bar
x := later
</pre></div>

<p>When a simply expanded variable is referenced, its value is substituted
verbatim.
</p>
<p>Here is a somewhat more complicated example, illustrating the use of
&lsquo;<samp>:=</samp>&rsquo; in conjunction with the <code>shell</code> function.
(See <a href="Functions.html#Shell-Function">The <code>shell</code> Function</a>.)  This example
also shows use of the variable <code>MAKELEVEL</code>, which is changed
when it is passed down from level to level.
(See <a href="Recipes.html#Variables_002fRecursion">Communicating Variables to a
Sub-<code>make</code></a>, for information about <code>MAKELEVEL</code>.)
</p>
<a name="index-MAKELEVEL-1"></a>
<a name="index-MAKE-1"></a>
<div class="example">
<pre class="example">ifeq (0,${MAKELEVEL})
whoami    := $(shell whoami)
host-type := $(shell arch)
MAKE := ${MAKE} host-type=${host-type} whoami=${whoami}
endif
</pre></div>

<p>An advantage of this use of &lsquo;<samp>:=</samp>&rsquo; is that a typical
&lsquo;descend into a directory&rsquo; recipe then looks like this:
</p>
<div class="example">
<pre class="example">${subdirs}:
        ${MAKE} -C $@ all
</pre></div>

<p>Simply expanded variables generally make complicated makefile programming
more predictable because they work like variables in most programming
languages.  They allow you to redefine a variable using its own value (or
its value processed in some way by one of the expansion functions) and to
use the expansion functions much more efficiently
(see <a href="Functions.html#Functions">Functions for Transforming Text</a>).
</p>
<a name="index-spaces_002c-in-variable-values"></a>
<a name="index-whitespace_002c-in-variable-values"></a>
<a name="index-variables_002c-spaces-in-values"></a>
<p>You can also use them to introduce controlled leading whitespace into
variable values.  Leading whitespace characters are discarded from your
input before substitution of variable references and function calls;
this means you can include leading spaces in a variable value by
protecting them with variable references, like this:
</p>
<div class="example">
<pre class="example">nullstring :=
space := $(nullstring) # end of the line
</pre></div>

<p>Here the value of the variable <code>space</code> is precisely one space.  The
comment &lsquo;<samp>#&nbsp;end&nbsp;of&nbsp;the&nbsp;line</samp>&rsquo;<!-- /@w --> is included here just for clarity.
Since trailing space characters are <em>not</em> stripped from variable
values, just a space at the end of the line would have the same effect
(but be rather hard to read).  If you put whitespace at the end of a
variable value, it is a good idea to put a comment like that at the end
of the line to make your intent clear.  Conversely, if you do <em>not</em>
want any whitespace characters at the end of your variable value, you
must remember not to put a random comment on the end of the line after
some whitespace, such as this:
</p>
<div class="example">
<pre class="example">dir := /foo/bar    # directory to put the frobs in
</pre></div>

<p>Here the value of the variable <code>dir</code> is &lsquo;<samp>/foo/bar&nbsp;&nbsp;&nbsp;&nbsp;</samp>&rsquo;<!-- /@w -->
(with four trailing spaces), which was probably not the intention.
(Imagine something like &lsquo;<samp>$(dir)/file</samp>&rsquo;<!-- /@w --> with this definition!)
</p>
<a name="index-conditional-variable-assignment"></a>
<a name="index-variables_002c-conditional-assignment"></a>
<a name="index-_003f_003d"></a>
<p>There is another assignment operator for variables, &lsquo;<samp>?=</samp>&rsquo;.  This
is called a conditional variable assignment operator, because it only
has an effect if the variable is not yet defined.  This statement:
</p>
<div class="example">
<pre class="example">FOO ?= bar
</pre></div>

<p>is exactly equivalent to this
(see <a href="Functions.html#Origin-Function">The <code>origin</code> Function</a>):
</p>
<div class="example">
<pre class="example">ifeq ($(origin FOO), undefined)
  FOO = bar
endif
</pre></div>

<p>Note that a variable set to an empty value is still defined, so
&lsquo;<samp>?=</samp>&rsquo; will not set that variable.
</p>
<hr>
<a name="Advanced"></a>
<div class="header">
<p>
Next: <a href="#Values" accesskey="n" rel="next">Values</a>, Previous: <a href="#Flavors" accesskey="p" rel="prev">Flavors</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Advanced-Features-for-Reference-to-Variables"></a>
<h3 class="section">6.3 Advanced Features for Reference to Variables</h3>
<a name="index-reference-to-variables-1"></a>

<p>This section describes some advanced features you can use to reference
variables in more flexible ways.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="#Substitution-Refs" accesskey="1">Substitution Refs</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Referencing a variable with
                                  substitutions on the value.
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="#Computed-Names" accesskey="2">Computed Names</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Computing the name of the variable to refer to.
</td></tr>
</table>

<hr>
<a name="Substitution-Refs"></a>
<div class="header">
<p>
Next: <a href="#Computed-Names" accesskey="n" rel="next">Computed Names</a>, Previous: <a href="#Advanced" accesskey="p" rel="prev">Advanced</a>, Up: <a href="#Advanced" accesskey="u" rel="up">Advanced</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Substitution-References"></a>
<h4 class="subsection">6.3.1 Substitution References</h4>
<a name="index-modified-variable-reference"></a>
<a name="index-substitution-variable-reference"></a>
<a name="index-variables_002c-modified-reference"></a>
<a name="index-variables_002c-substitution-reference"></a>

<a name="index-variables_002c-substituting-suffix-in"></a>
<a name="index-suffix_002c-substituting-in-variables"></a>
<p>A <em>substitution reference</em> substitutes the value of a variable with
alterations that you specify.  It has the form
&lsquo;<samp>$(<var>var</var>:<var>a</var>=<var>b</var>)</samp>&rsquo; (or
&lsquo;<samp>${<var>var</var>:<var>a</var>=<var>b</var>}</samp>&rsquo;) and its meaning is to take the value
of the variable <var>var</var>, replace every <var>a</var> at the end of a word with
<var>b</var> in that value, and substitute the resulting string.
</p>
<p>When we say &ldquo;at the end of a word&rdquo;, we mean that <var>a</var> must appear
either followed by whitespace or at the end of the value in order to be
replaced; other occurrences of <var>a</var> in the value are unaltered.  For
example:
</p>
<div class="example">
<pre class="example">foo := a.o b.o c.o
bar := $(foo:.o=.c)
</pre></div>

<p>sets &lsquo;<samp>bar</samp>&rsquo; to &lsquo;<samp>a.c b.c c.c</samp>&rsquo;.  See <a href="#Setting">Setting Variables</a>.
</p>
<p>A substitution reference is actually an abbreviation for use of the
<code>patsubst</code> expansion function (see <a href="Functions.html#Text-Functions">Functions for String Substitution and Analysis</a>).  We provide
substitution references as well as <code>patsubst</code> for compatibility with
other implementations of <code>make</code>.
</p>
<a name="index-patsubst"></a>
<p>Another type of substitution reference lets you use the full power of
the <code>patsubst</code> function.  It has the same form
&lsquo;<samp>$(<var>var</var>:<var>a</var>=<var>b</var>)</samp>&rsquo; described above, except that now
<var>a</var> must contain a single &lsquo;<samp>%</samp>&rsquo; character.  This case is
equivalent to &lsquo;<samp>$(patsubst <var>a</var>,<var>b</var>,$(<var>var</var>))</samp>&rsquo;.
See <a href="Functions.html#Text-Functions">Functions for String Substitution and Analysis</a>,
for a description of the <code>patsubst</code> function.
</p>
<div class="example">
<pre class="example">For example:
</pre><pre class="example">

foo := a.o b.o c.o
bar := $(foo:%.o=%.c)
</pre></div>

<p>sets &lsquo;<samp>bar</samp>&rsquo; to &lsquo;<samp>a.c b.c c.c</samp>&rsquo;.
</p>
<hr>
<a name="Computed-Names"></a>
<div class="header">
<p>
Previous: <a href="#Substitution-Refs" accesskey="p" rel="prev">Substitution Refs</a>, Up: <a href="#Advanced" accesskey="u" rel="up">Advanced</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Computed-Variable-Names"></a>
<h4 class="subsection">6.3.2 Computed Variable Names</h4>
<a name="index-nested-variable-reference"></a>
<a name="index-computed-variable-name"></a>
<a name="index-variables_002c-computed-names"></a>
<a name="index-variables_002c-nested-references"></a>
<a name="index-variables_002c-_0024-in-name"></a>
<a name="index-_0024_002c-in-variable-name"></a>
<a name="index-dollar-sign-_0028_0024_0029_002c-in-variable-name"></a>

<p>Computed variable names are a complicated concept needed only for
sophisticated makefile programming.  For most purposes you need not
consider them, except to know that making a variable with a dollar sign
in its name might have strange results.  However, if you are the type
that wants to understand everything, or you are actually interested in
what they do, read on.
</p>
<p>Variables may be referenced inside the name of a variable.  This is
called a <em>computed variable name</em> or a <em>nested variable
reference</em>.  For example,
</p>
<div class="example">
<pre class="example">x = y
y = z
a := $($(x))
</pre></div>

<p>defines <code>a</code> as &lsquo;<samp>z</samp>&rsquo;: the &lsquo;<samp>$(x)</samp>&rsquo; inside &lsquo;<samp>$($(x))</samp>&rsquo; expands
to &lsquo;<samp>y</samp>&rsquo;, so &lsquo;<samp>$($(x))</samp>&rsquo; expands to &lsquo;<samp>$(y)</samp>&rsquo; which in turn expands
to &lsquo;<samp>z</samp>&rsquo;.  Here the name of the variable to reference is not stated
explicitly; it is computed by expansion of &lsquo;<samp>$(x)</samp>&rsquo;.  The reference
&lsquo;<samp>$(x)</samp>&rsquo; here is nested within the outer variable reference.
</p>
<p>The previous example shows two levels of nesting, but any number of levels
is possible.  For example, here are three levels:
</p>
<div class="example">
<pre class="example">x = y
y = z
z = u
a := $($($(x)))
</pre></div>

<p>Here the innermost &lsquo;<samp>$(x)</samp>&rsquo; expands to &lsquo;<samp>y</samp>&rsquo;, so &lsquo;<samp>$($(x))</samp>&rsquo;
expands to &lsquo;<samp>$(y)</samp>&rsquo; which in turn expands to &lsquo;<samp>z</samp>&rsquo;; now we have
&lsquo;<samp>$(z)</samp>&rsquo;, which becomes &lsquo;<samp>u</samp>&rsquo;.
</p>
<p>References to recursively-expanded variables within a variable name are
re-expanded in the usual fashion.  For example:
</p>
<div class="example">
<pre class="example">x = $(y)
y = z
z = Hello
a := $($(x))
</pre></div>

<p>defines <code>a</code> as &lsquo;<samp>Hello</samp>&rsquo;: &lsquo;<samp>$($(x))</samp>&rsquo; becomes &lsquo;<samp>$($(y))</samp>&rsquo;
which becomes &lsquo;<samp>$(z)</samp>&rsquo; which becomes &lsquo;<samp>Hello</samp>&rsquo;.
</p>
<p>Nested variable references can also contain modified references and
function invocations (see <a href="Functions.html#Functions">Functions for Transforming Text</a>),
just like any other reference.
For example, using the <code>subst</code> function
(see <a href="Functions.html#Text-Functions">Functions for String Substitution and Analysis</a>):
</p>
<div class="example">
<pre class="example">x = variable1
variable2 := Hello
y = $(subst 1,2,$(x))
z = y
a := $($($(z)))
</pre></div>

<p>eventually defines <code>a</code> as &lsquo;<samp>Hello</samp>&rsquo;.  It is doubtful that anyone
would ever want to write a nested reference as convoluted as this one, but
it works: &lsquo;<samp>$($($(z)))</samp>&rsquo; expands to &lsquo;<samp>$($(y))</samp>&rsquo; which becomes
&lsquo;<samp>$($(subst 1,2,$(x)))</samp>&rsquo;.  This gets the value &lsquo;<samp>variable1</samp>&rsquo; from
<code>x</code> and changes it by substitution to &lsquo;<samp>variable2</samp>&rsquo;, so that the
entire string becomes &lsquo;<samp>$(variable2)</samp>&rsquo;, a simple variable reference
whose value is &lsquo;<samp>Hello</samp>&rsquo;.
</p>
<p>A computed variable name need not consist entirely of a single variable
reference.  It can contain several variable references, as well as some
invariant text.  For example,
</p>
<div class="example">
<pre class="example">a_dirs := dira dirb
1_dirs := dir1 dir2
</pre><pre class="example">

</pre><pre class="example">a_files := filea fileb
1_files := file1 file2
</pre><pre class="example">

</pre><pre class="example">ifeq &quot;$(use_a)&quot; &quot;yes&quot;
a1 := a
else
a1 := 1
endif
</pre><pre class="example">

</pre><pre class="example">ifeq &quot;$(use_dirs)&quot; &quot;yes&quot;
df := dirs
else
df := files
endif

dirs := $($(a1)_$(df))
</pre></div>

<p>will give <code>dirs</code> the same value as <code>a_dirs</code>, <code>1_dirs</code>,
<code>a_files</code> or <code>1_files</code> depending on the settings of <code>use_a</code>
and <code>use_dirs</code>.
</p>
<p>Computed variable names can also be used in substitution references:
</p>
<div class="example">
<pre class="example">a_objects := a.o b.o c.o
1_objects := 1.o 2.o 3.o

sources := $($(a1)_objects:.o=.c)
</pre></div>

<p>defines <code>sources</code> as either &lsquo;<samp>a.c b.c c.c</samp>&rsquo; or &lsquo;<samp>1.c 2.c 3.c</samp>&rsquo;,
depending on the value of <code>a1</code>.
</p>
<p>The only restriction on this sort of use of nested variable references
is that they cannot specify part of the name of a function to be called.
This is because the test for a recognized function name is done before
the expansion of nested references.  For example,
</p>
<div class="example">
<pre class="example">ifdef do_sort
func := sort
else
func := strip
endif
</pre><pre class="example">

</pre><pre class="example">bar := a d b g q c
</pre><pre class="example">

</pre><pre class="example">foo := $($(func) $(bar))
</pre></div>

<p>attempts to give &lsquo;<samp>foo</samp>&rsquo; the value of the variable &lsquo;<samp>sort a d b g
q c</samp>&rsquo; or &lsquo;<samp>strip a d b g q c</samp>&rsquo;, rather than giving &lsquo;<samp>a d b g q c</samp>&rsquo;
as the argument to either the <code>sort</code> or the <code>strip</code> function.
This restriction could be removed in the future if that change is shown
to be a good idea.
</p>
<p>You can also use computed variable names in the left-hand side of a
variable assignment, or in a <code>define</code> directive, as in:
</p>
<div class="example">
<pre class="example">dir = foo
$(dir)_sources := $(wildcard $(dir)/*.c)
define $(dir)_print =
lpr $($(dir)_sources)
endef
</pre></div>

<p>This example defines the variables &lsquo;<samp>dir</samp>&rsquo;, &lsquo;<samp>foo_sources</samp>&rsquo;, and
&lsquo;<samp>foo_print</samp>&rsquo;.
</p>
<p>Note that <em>nested variable references</em> are quite different from
<em>recursively expanded variables</em>
(see <a href="#Flavors">The Two Flavors of Variables</a>), though both are
used together in complex ways when doing makefile programming.
</p>
<hr>
<a name="Values"></a>
<div class="header">
<p>
Next: <a href="#Setting" accesskey="n" rel="next">Setting</a>, Previous: <a href="#Advanced" accesskey="p" rel="prev">Advanced</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="How-Variables-Get-Their-Values"></a>
<h3 class="section">6.4 How Variables Get Their Values</h3>
<a name="index-variables_002c-how-they-get-their-values"></a>
<a name="index-value_002c-how-a-variable-gets-it"></a>

<p>Variables can get values in several different ways:
</p>
<ul>
<li> You can specify an overriding value when you run <code>make</code>.
See <a href="Running.html#Overriding">Overriding Variables</a>.

</li><li> You can specify a value in the makefile, either
with an assignment (see <a href="#Setting">Setting Variables</a>) or with a
verbatim definition (see <a href="#Multi_002dLine">Defining Multi-Line Variables</a>).

</li><li> Variables in the environment become <code>make</code> variables.
See <a href="#Environment">Variables from the Environment</a>.

</li><li> Several <em>automatic</em> variables are given new values for each rule.
Each of these has a single conventional use.
See <a href="Implicit-Rules.html#Automatic-Variables">Automatic Variables</a>.

</li><li> Several variables have constant initial values.
See <a href="Implicit-Rules.html#Implicit-Variables">Variables Used by Implicit Rules</a>.
</li></ul>

<hr>
<a name="Setting"></a>
<div class="header">
<p>
Next: <a href="#Appending" accesskey="n" rel="next">Appending</a>, Previous: <a href="#Values" accesskey="p" rel="prev">Values</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Setting-Variables"></a>
<h3 class="section">6.5 Setting Variables</h3>
<a name="index-setting-variables"></a>
<a name="index-variables_002c-setting"></a>
<a name="index-_003d-1"></a>
<a name="index-_003a_003d-1"></a>
<a name="index-_003a_003a_003d-1"></a>
<a name="index-_003f_003d-1"></a>
<a name="index-_0021_003d"></a>

<p>To set a variable from the makefile, write a line starting with the
variable name followed by &lsquo;<samp>=</samp>&rsquo;, &lsquo;<samp>:=</samp>&rsquo;, or &lsquo;<samp>::=</samp>&rsquo;.  Whatever
follows the &lsquo;<samp>=</samp>&rsquo;, &lsquo;<samp>:=</samp>&rsquo;, or &lsquo;<samp>::=</samp>&rsquo; on the line becomes the
value.  For example,
</p>
<div class="example">
<pre class="example">objects = main.o foo.o bar.o utils.o
</pre></div>

<p>defines a variable named <code>objects</code>.  Whitespace around the variable
name and immediately after the &lsquo;<samp>=</samp>&rsquo; is ignored.
</p>
<p>Variables defined with &lsquo;<samp>=</samp>&rsquo; are <em>recursively expanded</em>
variables.  Variables defined with &lsquo;<samp>:=</samp>&rsquo; or &lsquo;<samp>::=</samp>&rsquo; are
<em>simply expanded</em> variables; these definitions can contain
variable references which will be expanded before the definition is
made.  See <a href="#Flavors">The Two Flavors of Variables</a>.
</p>
<p>The variable name may contain function and variable references, which
are expanded when the line is read to find the actual variable name to use.
</p>
<p>There is no limit on the length of the value of a variable except the
amount of memory on the computer.  You can split the value of a
variable into multiple physical lines for readability
(see <a href="Makefiles.html#Splitting-Lines">Splitting Long Lines</a>).
</p>
<p>Most variable names are considered to have the empty string as a value if
you have never set them.  Several variables have built-in initial values
that are not empty, but you can set them in the usual ways
(see <a href="Implicit-Rules.html#Implicit-Variables">Variables Used by Implicit Rules</a>).
Several special variables are set
automatically to a new value for each rule; these are called the
<em>automatic</em> variables (see <a href="Implicit-Rules.html#Automatic-Variables">Automatic Variables</a>).
</p>
<p>If you&rsquo;d like a variable to be set to a value only if it&rsquo;s not already
set, then you can use the shorthand operator &lsquo;<samp>?=</samp>&rsquo; instead of
&lsquo;<samp>=</samp>&rsquo;.  These two settings of the variable &lsquo;<samp>FOO</samp>&rsquo; are identical
(see <a href="Functions.html#Origin-Function">The <code>origin</code> Function</a>):
</p>
<div class="example">
<pre class="example">FOO ?= bar
</pre></div>

<p>and
</p>
<div class="example">
<pre class="example">ifeq ($(origin FOO), undefined)
FOO = bar
endif
</pre></div>

<p>The shell assignment operator &lsquo;<samp>!=</samp>&rsquo; can be used to execute a
shell script and set a variable to its output.  This operator first
evaluates the right-hand side, then passes that result to the shell
for execution.  If the result of the execution ends in a newline, that
one newline is removed; all other newlines are replaced by spaces.
The resulting string is then placed into the named
recursively-expanded variable.  For example:
</p>
<div class="example">
<pre class="example">hash != printf '\043'
file_list != find . -name '*.c'
</pre></div>

<p>If the result of the execution could produce a <code>$</code>, and you don&rsquo;t
intend what follows that to be interpreted as a make variable or
function reference, then you must replace every <code>$</code> with
<code>$$</code> as part of the execution.  Alternatively, you can set a
simply expanded variable to the result of running a program using the
<code>shell</code> function call.  See <a href="Functions.html#Shell-Function">The <code>shell</code>
Function</a>.  For example:
</p>
<div class="example">
<pre class="example">hash := $(shell printf '\043')
var := $(shell find . -name &quot;*.c&quot;)
</pre></div>

<p>As with the <code>shell</code> function, the exit status of the just-invoked
shell script is stored in the <code>.SHELLSTATUS</code> variable.
</p>

<hr>
<a name="Appending"></a>
<div class="header">
<p>
Next: <a href="#Override-Directive" accesskey="n" rel="next">Override Directive</a>, Previous: <a href="#Setting" accesskey="p" rel="prev">Setting</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Appending-More-Text-to-Variables"></a>
<h3 class="section">6.6 Appending More Text to Variables</h3>
<a name="index-_002b_003d"></a>
<a name="index-appending-to-variables"></a>
<a name="index-variables_002c-appending-to"></a>

<p>Often it is useful to add more text to the value of a variable already defined.
You do this with a line containing &lsquo;<samp>+=</samp>&rsquo;, like this:
</p>
<div class="example">
<pre class="example">objects += another.o
</pre></div>

<p>This takes the value of the variable <code>objects</code>, and adds the text
&lsquo;<samp>another.o</samp>&rsquo; to it (preceded by a single space).  Thus:
</p>
<div class="example">
<pre class="example">objects = main.o foo.o bar.o utils.o
objects += another.o
</pre></div>

<p>sets <code>objects</code> to &lsquo;<samp>main.o foo.o bar.o utils.o another.o</samp>&rsquo;.
</p>
<p>Using &lsquo;<samp>+=</samp>&rsquo; is similar to:
</p>
<div class="example">
<pre class="example">objects = main.o foo.o bar.o utils.o
objects := $(objects) another.o
</pre></div>

<p>but differs in ways that become important when you use more complex values.
</p>
<p>When the variable in question has not been defined before, &lsquo;<samp>+=</samp>&rsquo;
acts just like normal &lsquo;<samp>=</samp>&rsquo;: it defines a recursively-expanded
variable.  However, when there <em>is</em> a previous definition, exactly
what &lsquo;<samp>+=</samp>&rsquo; does depends on what flavor of variable you defined
originally.  See <a href="#Flavors">The Two Flavors of Variables</a>, for an
explanation of the two flavors of variables.
</p>
<p>When you add to a variable&rsquo;s value with &lsquo;<samp>+=</samp>&rsquo;, <code>make</code> acts
essentially as if you had included the extra text in the initial
definition of the variable.  If you defined it first with &lsquo;<samp>:=</samp>&rsquo; or
&lsquo;<samp>::=</samp>&rsquo;, making it a simply-expanded variable, &lsquo;<samp>+=</samp>&rsquo; adds to
that simply-expanded definition, and expands the new text before
appending it to the old value just as &lsquo;<samp>:=</samp>&rsquo; does (see
<a href="#Setting">Setting Variables</a>, for a full explanation of
&lsquo;<samp>:=</samp>&rsquo; or &lsquo;<samp>::=</samp>&rsquo;).  In fact,
</p>
<div class="example">
<pre class="example">variable := value
variable += more
</pre></div>

<p>is exactly equivalent to:
</p>
<div class="example">
<pre class="example">variable := value
variable := $(variable) more
</pre></div>

<p>On the other hand, when you use &lsquo;<samp>+=</samp>&rsquo; with a variable that you defined
first to be recursively-expanded using plain &lsquo;<samp>=</samp>&rsquo;, <code>make</code> does
something a bit different.  Recall that when you define a
recursively-expanded variable, <code>make</code> does not expand the value you set
for variable and function references immediately.  Instead it stores the text
verbatim, and saves these variable and function references to be expanded
later, when you refer to the new variable (see <a href="#Flavors">The Two Flavors
of Variables</a>).  When you use &lsquo;<samp>+=</samp>&rsquo; on a recursively-expanded variable,
it is this unexpanded text to which <code>make</code> appends the new text you
specify.
</p>
<div class="example">
<pre class="example">variable = value
variable += more
</pre></div>

<p>is roughly equivalent to:
</p>
<div class="example">
<pre class="example">temp = value
variable = $(temp) more
</pre></div>

<p>except that of course it never defines a variable called <code>temp</code>.
The importance of this comes when the variable&rsquo;s old value contains
variable references.  Take this common example:
</p>
<div class="example">
<pre class="example">CFLAGS = $(includes) -O
&hellip;
CFLAGS += -pg # enable profiling
</pre></div>

<p>The first line defines the <code>CFLAGS</code> variable with a reference to another
variable, <code>includes</code>.  (<code>CFLAGS</code> is used by the rules for C
compilation; see <a href="Implicit-Rules.html#Catalogue-of-Rules">Catalogue of Built-In Rules</a>.)
Using &lsquo;<samp>=</samp>&rsquo; for the definition makes <code>CFLAGS</code> a recursively-expanded
variable, meaning &lsquo;<samp>$(includes)&nbsp;<span class="nolinebreak">-O</span></samp>&rsquo;<!-- /@w --> is <em>not</em> expanded when
<code>make</code> processes the definition of <code>CFLAGS</code>.  Thus, <code>includes</code>
need not be defined yet for its value to take effect.  It only has to be
defined before any reference to <code>CFLAGS</code>.  If we tried to append to the
value of <code>CFLAGS</code> without using &lsquo;<samp>+=</samp>&rsquo;, we might do it like this:
</p>
<div class="example">
<pre class="example">CFLAGS := $(CFLAGS) -pg # enable profiling
</pre></div>

<p>This is pretty close, but not quite what we want.  Using &lsquo;<samp>:=</samp>&rsquo;
redefines <code>CFLAGS</code> as a simply-expanded variable; this means
<code>make</code> expands the text &lsquo;<samp>$(CFLAGS)&nbsp;<span class="nolinebreak">-pg</span></samp>&rsquo;<!-- /@w --> before setting the
variable.  If <code>includes</code> is not yet defined, we get &lsquo;<samp>&nbsp;<span class="nolinebreak">-O</span>&nbsp;<span class="nolinebreak">-pg</span></samp>&rsquo;<!-- /@w -->, and a later definition of <code>includes</code> will have no effect.
Conversely, by using &lsquo;<samp>+=</samp>&rsquo; we set <code>CFLAGS</code> to the
<em>unexpanded</em> value &lsquo;<samp>$(includes)&nbsp;<span class="nolinebreak">-O</span>&nbsp;<span class="nolinebreak">-pg</span></samp>&rsquo;<!-- /@w -->.  Thus we preserve
the reference to <code>includes</code>, so if that variable gets defined at
any later point, a reference like &lsquo;<samp>$(CFLAGS)</samp>&rsquo; still uses its
value.
</p>
<hr>
<a name="Override-Directive"></a>
<div class="header">
<p>
Next: <a href="#Multi_002dLine" accesskey="n" rel="next">Multi-Line</a>, Previous: <a href="#Appending" accesskey="p" rel="prev">Appending</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="The-override-Directive"></a>
<h3 class="section">6.7 The <code>override</code> Directive</h3>
<a name="index-override"></a>
<a name="index-overriding-with-override"></a>
<a name="index-variables_002c-overriding"></a>

<p>If a variable has been set with a command argument
(see <a href="Running.html#Overriding">Overriding Variables</a>),
then ordinary assignments in the makefile are ignored.  If you want to set
the variable in the makefile even though it was set with a command
argument, you can use an <code>override</code> directive, which is a line that
looks like this:
</p>
<div class="example">
<pre class="example">override <var>variable</var> = <var>value</var>
</pre></div>

<p>or
</p>
<div class="example">
<pre class="example">override <var>variable</var> := <var>value</var>
</pre></div>

<p>To append more text to a variable defined on the command line, use:
</p>
<div class="example">
<pre class="example">override <var>variable</var> += <var>more text</var>
</pre></div>

<p>See <a href="#Appending">Appending More Text to Variables</a>.
</p>
<p>Variable assignments marked with the <code>override</code> flag have a
higher priority than all other assignments, except another
<code>override</code>.  Subsequent assignments or appends to this variable
which are not marked <code>override</code> will be ignored.
</p>
<p>The <code>override</code> directive was not invented for escalation in the war
between makefiles and command arguments.  It was invented so you can alter
and add to values that the user specifies with command arguments.
</p>
<p>For example, suppose you always want the &lsquo;<samp>-g</samp>&rsquo; switch when you run the
C compiler, but you would like to allow the user to specify the other
switches with a command argument just as usual.  You could use this
<code>override</code> directive:
</p>
<div class="example">
<pre class="example">override CFLAGS += -g
</pre></div>

<p>You can also use <code>override</code> directives with <code>define</code> directives.
This is done as you might expect:
</p>
<div class="example">
<pre class="example">override define foo =
bar
endef
</pre></div>

<p>See <a href="#Multi_002dLine">Defining Multi-Line Variables</a>.
</p>
<hr>
<a name="Multi_002dLine"></a>
<div class="header">
<p>
Next: <a href="#Undefine-Directive" accesskey="n" rel="next">Undefine Directive</a>, Previous: <a href="#Override-Directive" accesskey="p" rel="prev">Override Directive</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Defining-Multi_002dLine-Variables"></a>
<h3 class="section">6.8 Defining Multi-Line Variables</h3>
<a name="index-define"></a>
<a name="index-endef"></a>
<a name="index-multi_002dline-variable-definition"></a>
<a name="index-variables_002c-multi_002dline"></a>
<a name="index-verbatim-variable-definition"></a>
<a name="index-defining-variables-verbatim"></a>
<a name="index-variables_002c-defining-verbatim"></a>

<p>Another way to set the value of a variable is to use the <code>define</code>
directive.  This directive has an unusual syntax which allows newline
characters to be included in the value, which is convenient for
defining both canned sequences of commands (see <a href="Recipes.html#Canned-Recipes">Defining Canned Recipes</a>), and also sections of makefile syntax to
use with <code>eval</code> (see <a href="Functions.html#Eval-Function">Eval Function</a>).
</p>
<p>The <code>define</code> directive is followed on the same line by the name
of the variable being defined and an (optional) assignment operator,
and nothing more.  The value to give the variable appears on the
following lines.  The end of the value is marked by a line containing
just the word <code>endef</code>.  Aside from this difference in syntax,
<code>define</code> works just like any other variable definition.  The
variable name may contain function and variable references, which are
expanded when the directive is read to find the actual variable name
to use.
</p>
<p>You may omit the variable assignment operator if you prefer.  If
omitted, <code>make</code> assumes it to be &lsquo;<samp>=</samp>&rsquo; and creates a
recursively-expanded variable (see <a href="#Flavors">The Two Flavors of Variables</a>).
When using a &lsquo;<samp>+=</samp>&rsquo; operator, the value is appended to the previous
value as with any other append operation: with a single space
separating the old and new values.
</p>
<p>You may nest <code>define</code> directives: <code>make</code> will keep track of
nested directives and report an error if they are not all properly
closed with <code>endef</code>.  Note that lines beginning with the recipe
prefix character are considered part of a recipe, so any <code>define</code>
or <code>endef</code> strings appearing on such a line will not be
considered <code>make</code> directives.
</p>
<div class="example">
<pre class="example">define two-lines =
echo foo
echo $(bar)
endef
</pre></div>

<p>The value in an ordinary assignment cannot contain a newline; but the
newlines that separate the lines of the value in a <code>define</code> become
part of the variable&rsquo;s value (except for the final newline which precedes
the <code>endef</code> and is not considered part of the value).
</p>
<p>When used in a recipe, the previous example is functionally equivalent
to this:
</p>
<div class="example">
<pre class="example">two-lines = echo foo; echo $(bar)
</pre></div>

<p>since two commands separated by semicolon behave much like two separate
shell commands.  However, note that using two separate lines means
<code>make</code> will invoke the shell twice, running an independent sub-shell
for each line.  See <a href="Recipes.html#Execution">Recipe Execution</a>.
</p>
<p>If you want variable definitions made with <code>define</code> to take
precedence over command-line variable definitions, you can use the
<code>override</code> directive together with <code>define</code>:
</p>
<div class="example">
<pre class="example">override define two-lines =
foo
$(bar)
endef
</pre></div>

<p>See <a href="#Override-Directive">The <code>override</code> Directive</a>.
</p>
<hr>
<a name="Undefine-Directive"></a>
<div class="header">
<p>
Next: <a href="#Environment" accesskey="n" rel="next">Environment</a>, Previous: <a href="#Multi_002dLine" accesskey="p" rel="prev">Multi-Line</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Undefining-Variables"></a>
<h3 class="section">6.9 Undefining Variables</h3>
<a name="index-undefine"></a>
<a name="index-undefining-variable"></a>

<p>If you want to clear a variable, setting its value to empty is usually
sufficient. Expanding such a variable will yield the same result (empty
string) regardless of whether it was set or not. However, if you are
using the <code>flavor</code> (see <a href="Functions.html#Flavor-Function">Flavor Function</a>) and
<code>origin</code> (see <a href="Functions.html#Origin-Function">Origin Function</a>) functions, there is a difference
between a variable that was never set and a variable with an empty value.
In such situations you may want to use the <code>undefine</code> directive to
make a variable appear as if it was never set. For example:
</p>
<div class="example">
<pre class="example">foo := foo
bar = bar

undefine foo
undefine bar

$(info $(origin foo))
$(info $(flavor bar))
</pre></div>

<p>This example will print &ldquo;undefined&rdquo; for both variables.
</p>
<p>If you want to undefine a command-line variable definition, you can use
the <code>override</code> directive together with <code>undefine</code>, similar to
how this is done for variable definitions:
</p>
<div class="example">
<pre class="example">override undefine CFLAGS
</pre></div>

<hr>
<a name="Environment"></a>
<div class="header">
<p>
Next: <a href="#Target_002dspecific" accesskey="n" rel="next">Target-specific</a>, Previous: <a href="#Undefine-Directive" accesskey="p" rel="prev">Undefine Directive</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Variables-from-the-Environment"></a>
<h3 class="section">6.10 Variables from the Environment</h3>

<a name="index-variables_002c-environment-1"></a>
<a name="index-environment"></a>
<p>Variables in <code>make</code> can come from the environment in which
<code>make</code> is run.  Every environment variable that <code>make</code> sees
when it starts up is transformed into a <code>make</code> variable with the
same name and value.  However, an explicit assignment in the makefile,
or with a command argument, overrides the environment.  (If the
&lsquo;<samp>-e</samp>&rsquo; flag is specified, then values from the environment override
assignments in the makefile.  See <a href="Running.html#Options-Summary">Summary of
Options</a>.  But this is not recommended practice.)
</p>
<p>Thus, by setting the variable <code>CFLAGS</code> in your environment, you can
cause all C compilations in most makefiles to use the compiler switches you
prefer.  This is safe for variables with standard or conventional meanings
because you know that no makefile will use them for other things.  (Note
this is not totally reliable; some makefiles set <code>CFLAGS</code> explicitly
and therefore are not affected by the value in the environment.)
</p>
<p>When <code>make</code> runs a recipe, variables defined in the
makefile are placed into the environment of each shell.  This allows
you to pass values to sub-<code>make</code> invocations (see <a href="Recipes.html#Recursion">Recursive Use of <code>make</code></a>).  By default, only variables that came
from the environment or the command line are passed to recursive
invocations.  You can use the <code>export</code> directive to pass other
variables.  See <a href="Recipes.html#Variables_002fRecursion">Communicating Variables to a
Sub-<code>make</code></a>, for full details.
</p>
<p>Other use of variables from the environment is not recommended.  It is not
wise for makefiles to depend for their functioning on environment variables
set up outside their control, since this would cause different users to get
different results from the same makefile.  This is against the whole
purpose of most makefiles.
</p>
<a name="index-SHELL_002c-import-from-environment"></a>
<p>Such problems would be especially likely with the variable
<code>SHELL</code>, which is normally present in the environment to specify
the user&rsquo;s choice of interactive shell.  It would be very undesirable
for this choice to affect <code>make</code>; so, <code>make</code> handles the
<code>SHELL</code> environment variable in a special way; see <a href="Recipes.html#Choosing-the-Shell">Choosing the Shell</a>.
</p>
<hr>
<a name="Target_002dspecific"></a>
<div class="header">
<p>
Next: <a href="#Pattern_002dspecific" accesskey="n" rel="next">Pattern-specific</a>, Previous: <a href="#Environment" accesskey="p" rel="prev">Environment</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Target_002dspecific-Variable-Values"></a>
<h3 class="section">6.11 Target-specific Variable Values</h3>
<a name="index-target_002dspecific-variables"></a>
<a name="index-variables_002c-target_002dspecific"></a>

<p>Variable values in <code>make</code> are usually global; that is, they are the
same regardless of where they are evaluated (unless they&rsquo;re reset, of
course).  One exception to that is automatic variables
(see <a href="Implicit-Rules.html#Automatic-Variables">Automatic Variables</a>).
</p>
<p>The other exception is <em>target-specific variable values</em>.  This
feature allows you to define different values for the same variable,
based on the target that <code>make</code> is currently building.  As with
automatic variables, these values are only available within the context
of a target&rsquo;s recipe (and in other target-specific assignments).
</p>
<p>Set a target-specific variable value like this:
</p>
<div class="example">
<pre class="example"><var>target</var> &hellip; : <var>variable-assignment</var>
</pre></div>

<p>Target-specific variable assignments can be prefixed with any or all of the
special keywords <code>export</code>, <code>override</code>, or <code>private</code>;
these apply their normal behavior to this instance of the variable only.
</p>
<p>Multiple <var>target</var> values create a target-specific variable value for
each member of the target list individually.
</p>
<p>The <var>variable-assignment</var> can be any valid form of assignment;
recursive (&lsquo;<samp>=</samp>&rsquo;), simple (&lsquo;<samp>:=</samp>&rsquo; or &lsquo;<samp>::=</samp>&rsquo;), appending
(&lsquo;<samp>+=</samp>&rsquo;), or conditional (&lsquo;<samp>?=</samp>&rsquo;).  All variables that appear
within the <var>variable-assignment</var> are evaluated within the context
of the target: thus, any previously-defined target-specific variable
values will be in effect.  Note that this variable is actually
distinct from any &ldquo;global&rdquo; value: the two variables do not have to
have the same flavor (recursive vs. simple).
</p>
<p>Target-specific variables have the same priority as any other makefile
variable.  Variables provided on the command line (and in the
environment if the &lsquo;<samp>-e</samp>&rsquo; option is in force) will take precedence.
Specifying the <code>override</code> directive will allow the target-specific
variable value to be preferred.
</p>
<p>There is one more special feature of target-specific variables: when
you define a target-specific variable that variable value is also in
effect for all prerequisites of this target, and all their
prerequisites, etc. (unless those prerequisites override that variable
with their own target-specific variable value).  So, for example, a
statement like this:
</p>
<div class="example">
<pre class="example">prog : CFLAGS = -g
prog : prog.o foo.o bar.o
</pre></div>

<p>will set <code>CFLAGS</code> to &lsquo;<samp>-g</samp>&rsquo; in the recipe for <samp>prog</samp>, but
it will also set <code>CFLAGS</code> to &lsquo;<samp>-g</samp>&rsquo; in the recipes that create
<samp>prog.o</samp>, <samp>foo.o</samp>, and <samp>bar.o</samp>, and any recipes which
create their prerequisites.
</p>
<p>Be aware that a given prerequisite will only be built once per
invocation of make, at most.  If the same file is a prerequisite of
multiple targets, and each of those targets has a different value for
the same target-specific variable, then the first target to be built
will cause that prerequisite to be built and the prerequisite will
inherit the target-specific value from the first target.  It will
ignore the target-specific values from any other targets.
</p>
<hr>
<a name="Pattern_002dspecific"></a>
<div class="header">
<p>
Next: <a href="#Suppressing-Inheritance" accesskey="n" rel="next">Suppressing Inheritance</a>, Previous: <a href="#Target_002dspecific" accesskey="p" rel="prev">Target-specific</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Pattern_002dspecific-Variable-Values"></a>
<h3 class="section">6.12 Pattern-specific Variable Values</h3>
<a name="index-pattern_002dspecific-variables"></a>
<a name="index-variables_002c-pattern_002dspecific"></a>

<p>In addition to target-specific variable values
(see <a href="#Target_002dspecific">Target-specific Variable Values</a>), GNU
<code>make</code> supports pattern-specific variable values.  In this form,
the variable is defined for any target that matches the pattern
specified.
</p>
<p>Set a pattern-specific variable value like this:
</p>
<div class="example">
<pre class="example"><var>pattern</var> &hellip; : <var>variable-assignment</var>
</pre></div>
<p>where <var>pattern</var> is a %-pattern.  As with target-specific variable
values, multiple <var>pattern</var> values create a pattern-specific variable
value for each pattern individually.  The <var>variable-assignment</var> can
be any valid form of assignment.  Any command line variable setting will
take precedence, unless <code>override</code> is specified.
</p>
<p>For example:
</p>
<div class="example">
<pre class="example">%.o : CFLAGS = -O
</pre></div>

<p>will assign <code>CFLAGS</code> the value of &lsquo;<samp>-O</samp>&rsquo; for all targets
matching the pattern <code>%.o</code>.
</p>
<p>If a target matches more than one pattern, the matching pattern-specific
variables with longer stems are interpreted first. This results in more
specific variables taking precedence over the more generic ones, for
example:
</p>
<div class="example">
<pre class="example">%.o: %.c
        $(CC) -c $(CFLAGS) $(CPPFLAGS) $&lt; -o $@

lib/%.o: CFLAGS := -fPIC -g
%.o: CFLAGS := -g

all: foo.o lib/bar.o
</pre></div>

<p>In this example the first definition of the <code>CFLAGS</code> variable
will be used to update <samp>lib/bar.o</samp> even though the second one
also applies to this target. Pattern-specific variables which result
in the same stem length are considered in the order in which they
were defined in the makefile.
</p>
<p>Pattern-specific variables are searched after any target-specific
variables defined explicitly for that target, and before target-specific
variables defined for the parent target.
</p>
<hr>
<a name="Suppressing-Inheritance"></a>
<div class="header">
<p>
Next: <a href="#Special-Variables" accesskey="n" rel="next">Special Variables</a>, Previous: <a href="#Pattern_002dspecific" accesskey="p" rel="prev">Pattern-specific</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Suppressing-Inheritance-1"></a>
<h3 class="section">6.13 Suppressing Inheritance</h3>
<a name="index-private"></a>
<a name="index-suppressing-inheritance"></a>
<a name="index-inheritance_002c-suppressing"></a>

<p>As described in previous sections, <code>make</code> variables are inherited
by prerequisites.  This capability allows you to modify the behavior
of a prerequisite based on which targets caused it to be rebuilt.  For
example, you might set a target-specific variable on a <code>debug</code>
target, then running &lsquo;<samp>make debug</samp>&rsquo; will cause that variable to be
inherited by all prerequisites of <code>debug</code>, while just running
&lsquo;<samp>make all</samp>&rsquo; (for example) would not have that assignment.
</p>
<p>Sometimes, however, you may not want a variable to be inherited.  For
these situations, <code>make</code> provides the <code>private</code> modifier.
Although this modifier can be used with any variable assignment, it
makes the most sense with target- and pattern-specific variables.  Any
variable marked <code>private</code> will be visible to its local target but
will not be inherited by prerequisites of that target.  A global
variable marked <code>private</code> will be visible in the global scope but
will not be inherited by any target, and hence will not be visible
in any recipe.
</p>
<p>As an example, consider this makefile:
</p><div class="example">
<pre class="example">EXTRA_CFLAGS =

prog: private EXTRA_CFLAGS = -L/usr/local/lib
prog: a.o b.o
</pre></div>

<p>Due to the <code>private</code> modifier, <code>a.o</code> and <code>b.o</code> will not
inherit the <code>EXTRA_CFLAGS</code> variable assignment from the
<code>prog</code> target.
</p>
<hr>
<a name="Special-Variables"></a>
<div class="header">
<p>
Previous: <a href="#Suppressing-Inheritance" accesskey="p" rel="prev">Suppressing Inheritance</a>, Up: <a href="#Using-Variables" accesskey="u" rel="up">Using Variables</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<a name="Other-Special-Variables"></a>
<h3 class="section">6.14 Other Special Variables</h3>
<a name="index-makefiles_002c-and-special-variables"></a>
<a name="index-special-variables"></a>

<p>GNU <code>make</code> supports some variables that have special properties.
</p>
<dl compact="compact">
<dd>
<a name="index-MAKEFILE_005fLIST-_0028list-of-parsed-makefiles_0029"></a>
<a name="index-makefiles_002c-and-MAKEFILE_005fLIST-variable"></a>
<a name="index-including-_0028MAKEFILE_005fLIST-variable_0029"></a>
</dd>
<dt><code>MAKEFILE_LIST</code></dt>
<dd><p>Contains the name of each makefile that is parsed by <code>make</code>, in
the order in which it was parsed.  The name is appended just
before <code>make</code> begins to parse the makefile.  Thus, if the first
thing a makefile does is examine the last word in this variable, it
will be the name of the current makefile.  Once the current makefile
has used <code>include</code>, however, the last word will be the
just-included makefile.
</p>
<p>If a makefile named <code>Makefile</code> has this content:
</p>
<div class="example">
<pre class="example">name1 := $(lastword $(MAKEFILE_LIST))

include inc.mk

name2 := $(lastword $(MAKEFILE_LIST))

all:
        @echo name1 = $(name1)
        @echo name2 = $(name2)
</pre></div>

<p>then you would expect to see this output:
</p>
<div class="example">
<pre class="example">name1 = Makefile
name2 = inc.mk
</pre></div>

<a name="index-_002eDEFAULT_005fGOAL-_0028define-default-goal_0029"></a>
</dd>
<dt><code>.DEFAULT_GOAL</code></dt>
<dd><p>Sets the default goal to be used if no targets were specified on the
command line (see <a href="Running.html#Goals">Arguments to Specify the Goals</a>).  The
<code>.DEFAULT_GOAL</code> variable allows you to discover the current
default goal, restart the default goal selection algorithm by clearing
its value, or to explicitly set the default goal.  The following
example illustrates these cases:
</p>
<div class="example">
<pre class="example"># Query the default goal.
ifeq ($(.DEFAULT_GOAL),)
  $(warning no default goal is set)
endif

.PHONY: foo
foo: ; @echo $@

$(warning default goal is $(.DEFAULT_GOAL))

# Reset the default goal.
.DEFAULT_GOAL :=

.PHONY: bar
bar: ; @echo $@

$(warning default goal is $(.DEFAULT_GOAL))

# Set our own.
.DEFAULT_GOAL := foo
</pre></div>

<p>This makefile prints:
</p>
<div class="example">
<pre class="example">no default goal is set
default goal is foo
default goal is bar
foo
</pre></div>

<p>Note that assigning more than one target name to <code>.DEFAULT_GOAL</code> is
invalid and will result in an error.
</p>
<a name="index-MAKE_005fRESTARTS-_0028number-of-times-make-has-restarted_0029"></a>
</dd>
<dt><code>MAKE_RESTARTS</code></dt>
<dd><p>This variable is set only if this instance of <code>make</code> has
restarted (see <a href="Makefiles.html#Remaking-Makefiles">How Makefiles Are Remade</a>): it
will contain the number of times this instance has restarted.  Note
this is not the same as recursion (counted by the <code>MAKELEVEL</code>
variable).  You should not set, modify, or export this variable.
</p>
<a name="index-MAKE_005fTERMOUT-_0028whether-stdout-is-a-terminal_0029"></a>
<a name="index-MAKE_005fTERMERR-_0028whether-stderr-is-a-terminal_0029"></a>
</dd>
<dt><code>MAKE_TERMOUT</code></dt>
<dt><code>MAKE_TERMERR</code></dt>
<dd><p>When <code>make</code> starts it will check whether stdout and stderr will
show their output on a terminal.  If so, it will set
<code>MAKE_TERMOUT</code> and <code>MAKE_TERMERR</code>, respectively, to the name
of the terminal device (or <code>true</code> if this cannot be determined).
If set these variables will be marked for export.  These variables
will not be changed by <code>make</code> and they will not be modified if
already set.
</p>
<p>These values can be used (particularly in combination with output
synchronization (see <a href="Recipes.html#Parallel-Output">Output During Parallel
Execution</a>) to determine whether <code>make</code> itself is writing to a
terminal; they can be tested to decide whether to force recipe
commands to generate colorized output for example.
</p>
<p>If you invoke a sub-<code>make</code> and redirect its stdout or stderr it
is your responsibility to reset or unexport these variables as well,
if your makefiles rely on them.
</p>
<a name="index-_002eRECIPEPREFIX-_0028change-the-recipe-prefix-character_0029"></a>
</dd>
<dt><code>.RECIPEPREFIX</code></dt>
<dd><p>The first character of the value of this variable is used as the
character make assumes is introducing a recipe line.  If the variable
is empty (as it is by default) that character is the standard tab
character.  For example, this is a valid makefile:
</p>
<div class="example">
<pre class="example">.RECIPEPREFIX = &gt;
all:
&gt; @echo Hello, world
</pre></div>

<p>The value of <code>.RECIPEPREFIX</code> can be changed multiple times; once set
it stays in effect for all rules parsed until it is modified.
</p>
<a name="index-_002eVARIABLES-_0028list-of-variables_0029"></a>
</dd>
<dt><code>.VARIABLES</code></dt>
<dd><p>Expands to a list of the <em>names</em> of all global variables defined
so far.  This includes variables which have empty values, as well as
built-in variables (see <a href="Implicit-Rules.html#Implicit-Variables">Variables Used by
Implicit Rules</a>), but does not include any variables which are only
defined in a target-specific context.  Note that any value you assign
to this variable will be ignored; it will always return its special
value.
</p>

<a name="index-_002eFEATURES-_0028list-of-supported-features_0029"></a>
</dd>
<dt><code>.FEATURES</code></dt>
<dd><p>Expands to a list of special features supported by this version of
<code>make</code>.  Possible values include, but are not limited to:
</p>
<dl compact="compact">
<dt>&lsquo;<samp>archives</samp>&rsquo;</dt>
<dd><p>Supports <code>ar</code> (archive) files using special file name syntax.
See <a href="Archives.html#Archives">Using <code>make</code> to Update Archive Files</a>.
</p>
</dd>
<dt>&lsquo;<samp>check-symlink</samp>&rsquo;</dt>
<dd><p>Supports the <code>-L</code> (<code>--check-symlink-times</code>) flag.
See <a href="Running.html#Options-Summary">Summary of Options</a>.
</p>
</dd>
<dt>&lsquo;<samp>else-if</samp>&rsquo;</dt>
<dd><p>Supports &ldquo;else if&rdquo; non-nested conditionals.  See <a href="Conditionals.html#Conditional-Syntax">Syntax of Conditionals</a>.
</p>
</dd>
<dt>&lsquo;<samp>jobserver</samp>&rsquo;</dt>
<dd><p>Supports &ldquo;job server&rdquo; enhanced parallel builds.  See <a href="Recipes.html#Parallel">Parallel Execution</a>.
</p>
</dd>
<dt>&lsquo;<samp>oneshell</samp>&rsquo;</dt>
<dd><p>Supports the <code>.ONESHELL</code> special target.  See <a href="Recipes.html#One-Shell">Using
One Shell</a>.
</p>
</dd>
<dt>&lsquo;<samp>order-only</samp>&rsquo;</dt>
<dd><p>Supports order-only prerequisites.  See <a href="Rules.html#Prerequisite-Types">Types
of Prerequisites</a>.
</p>
</dd>
<dt>&lsquo;<samp>second-expansion</samp>&rsquo;</dt>
<dd><p>Supports secondary expansion of prerequisite lists.
</p>
</dd>
<dt>&lsquo;<samp>shortest-stem</samp>&rsquo;</dt>
<dd><p>Uses the &ldquo;shortest stem&rdquo; method of choosing which pattern, of
multiple applicable options, will be used.  See <a href="Implicit-Rules.html#Pattern-Match">How
Patterns Match</a>.
</p>
</dd>
<dt>&lsquo;<samp>target-specific</samp>&rsquo;</dt>
<dd><p>Supports target-specific and pattern-specific variable assignments.
See <a href="#Target_002dspecific">Target-specific Variable Values</a>.
</p>
</dd>
<dt>&lsquo;<samp>undefine</samp>&rsquo;</dt>
<dd><p>Supports the <code>undefine</code> directive.  See <a href="#Undefine-Directive">Undefine Directive</a>.
</p>
</dd>
<dt>&lsquo;<samp>guile</samp>&rsquo;</dt>
<dd><p>Has GNU Guile available as an embedded extension language.
See <a href="Extending-make.html#Guile-Integration">GNU Guile Integration</a>.
</p>
</dd>
<dt>&lsquo;<samp>load</samp>&rsquo;</dt>
<dd><p>Supports dynamically loadable objects for creating custom extensions.
See <a href="Extending-make.html#Loading-Objects">Loading Dynamic Objects</a>.
</p></dd>
</dl>

<a name="index-_002eINCLUDE_005fDIRS-_0028list-of-include-directories_0029"></a>
</dd>
<dt><code>.INCLUDE_DIRS</code></dt>
<dd><p>Expands to a list of directories that <code>make</code> searches for
included makefiles (see <a href="Makefiles.html#Include">Including Other Makefiles</a>).
</p>
</dd>
</dl>

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